The present invention relates to a radiological device with a radiogenic tube of the magnetic bearing type, applicable to the general field of radio diagnosis.
In radiological devices X rays are produced from the rotating anode of a radiogenic tube. The anode is rotated by a drive motor comprising a rotor disposed inside the radiogenic tube along the rotational axis of the anode, and including a stator disposed concentrically with the rotor and outside the envelope of the radiogenic tube.
The high voltage required for operation of the radiogenic tube, supplied by a high voltage generator, is brought to the radiogenic tube by cables having a high level of electrical insulation; the negative polarity being applied to the cathode of the radiogenic tube and the positive polarity being applied to the anode.
Generally, the negative polarity is referenced to ground or the mass, as well as a protective sheath containing the radiogenic tube. The cable feeding the positive polarity penetrates into this sheath, in which it is connected to a connecting pin of the radiogenic tube, this pin is electrically connected to the anode.
For the motors driving the anode in rotation, whose rotor is suspended by mechanical means, such as ball bearings, it is conventional to apply the positive polarity of the high voltage to the rotor, this positive polarity is transmitted to the rotating anode through a metal shaft connecting the rotor and the anode together; the stator is fed at a low voltage reference to ground. We then find between the stator and the rotor the whole value of the positive high voltage, this value reaching high levels (85 KV). The electrical insulation is ensured by an assembly of dielectrics formed more particularly by the glass of the envelope of the radiogenic tube, an insulating oil and an insulating screen, the vacuum formed in the radiogenic tube forming an additional insulation. This assembly of dielectrics passes more particularly into the space between the stator and the rotor, which space forms the air gap of the drive motor for the anode.
Drive motors of the type whose rotor rotates on the ball bearings have the drawback of relatively rapid deterioration due to the mechanical wear of these ball bearings. Thus, manufactures tend to replace this type of mechanical suspension by suspensions of the magnetic bearing type.
Magnetic bearings are formed by an assembly of stators, each having a certain number of independent electromagnets, through which a controlled current flows so that the rotor is balanced under the influence of the electromagnetic forces. This balanced position, called nominal position, is determined by means of detectors which permanently record the position of the rotor. The signals generated by these detectors allow the current flowing through the electromagnets to be modified automatically and consequently the force of the magnetic fields. The current modification occurs through an electronic servocontrol system to which each electromagnet and each detector are connected.
A considerable drawback to the use of magnetic bearing motors resides in the large number of electric connections (a few tens of conductors) which connect the servo control electronics to the elements which it controls, which form a hindrance since the sheath containing the radiogenic tube is generally mobile.
On the other hand, the correct operation of the magnetic bearing motor requires the air gap (between the rotor and the stator) to be very small, which makes it difficult or impossible to provide the necessary electric insulation between the stators and the rotors with respect to the high voltage.
A known solution consists in isolating the rotor electrically with respect to the rotating anode: since the rotating anode is connected to the positive pole of the high voltage, it is mechanically secured to the rotor, for its rotation, by means of an insulating shaft made for example from alumina; the rotor is then referenced to the ground as well as the stators.
This solution to the problem of electric insulation comprises a serious drawback which resides in the mechanical fragility of this insulating shaft, considering the high rotational speeds and the high temperatures to which this insulating shaft is subjected.